Accurate control and readout of the optical frequency of a laser is important for many applications such as high-resolution spectroscopy, time and frequency metrology, interferometry and length metrology, optical-based communication, and optical component characterization. Of current importance is the test and measurement of the wavelength dependence of the properties of optical networking components. Components for fiber-optic networks—such as optical filters, couplers, interleavers, and the like—have critical specifications for insertion loss (IL), polarization dependent loss (PDL), polarization mode dispersion (PMD) and other properties.
Each of these properties depends on the optical wavelength. In a typical application, light from a laser is inserted into a component under test and the transmission and/or reflection properties of the device are recorded as the wavelength of the laser is swept over a range of wavelengths. Accurate knowledge of the wavelength, or optical frequency, which is acquired in near real-time during a wavelength sweep, is of utmost importance for measuring the properties of optical networking components, and in fact will be a critical technology as the channel spacings in fiber-optic networks shrink to accommodate more bandwidth per optical fiber.
Many high-resolution techniques exist for very accurate control of laser frequency sweeps, but these techniques are usually for laboratory experiments where size, cost and complexity of the technique are not of primary importance. A further disadvantage of these techniques is that they are extremely limited in their operational wavelength range and are not useful for characterizing optical networking components. Commercial wavelength meters are better in that they can accurately measure wavelength over a very large wavelength range. However, commercial meters are too expensive and too large to fit into other test instruments. The measurement rates of commercial wavelength meters are also much too slow (1-10 Hz) for the rapid rates of characterization of optical components (1-100 kHz).
There is a need for a wavelength meter that generates accurate real-time measurements and readouts of the optical frequency of a swept, tunable laser. The readout rate must be fast enough to accommodate rapid frequency scans, and accurate over wide optical frequency regions. There is need for the meter to be very compact to fit inside other instruments or small form factors. The meter must be robust and inexpensive.
There is also need for accurate feedback control of the optical frequency of a swept laser source. The optical frequency meter of the feedback monitor must be very compact, inexpensive, robust, fast and accurate over wide frequency ranges.